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Mammalian dosage compensation of X-linked genes is achieved between XX females and XY males by silencing one of the two X chromosomes. It is the expression of a functional non-coding RNA transcript, XIST that is responsible for the initiation of silencing during X-chromosome inactivation. XIST expression and subsequent in cis localization to the future inactive X chromosome initiates a cascade of epigenetic events that leads to the formation of facultative heterochromatin. The exact role the XIST RNA plays in establishing and maintaining the inactive state is uncertain.
It is hypothesized that the XIST RNA sets up a repressive nuclear compartment and transcriptionally silences through the recruitment of factors required for setting up the heterochromatic state of the inactive X. In this thesis I address XIST/Xist’s recruitment of factors with two separate approaches: 1) I ask whether Xist, in the absence of silencing is able to recruit epigenetic marks in a somatic cell hybrid system, 2) I evaluate a system whereby the XIST RNA is tagged and can be isolated to identify novel RNA-protein interactions.
To assess epigenetic features that may be directly recruited by the expression of the XIST/Xist RNA, I have analyzed mouse/human somatic cell hybrids where XIST/Xist expression and silencing are disconnected. Loss of active chromatin marks, H3 acetylation and H3 lysine 4 methylation, was not observed with XIST/Xist expression; nor was there a gain of DNA methylation or the silencing of the Cot-1 fraction. Therefore, these marks of heterochromatin are not solely dependent upon XIST/Xist expression in a somatic cell.
The isolation of the XIST RNA with its interacting partners would allow us to better understand the mechanism by which XIST acts to silence the inactive X. I have developed a MS2 stem loop-tagged XIST RNA and integrated it into an inducible XIST system. The MS2 tagging system is a valid system for pulling out the XIST RNA and identifying interacting proteins since the tagged RNA was able to interact with whatever factor(s) are required to silence the Cot-1 fraction and form a localized RNA signal.